Pneumatic tires are often provided with a circumferential tread of a cap/base configuration comprised of an outer ground-contacting tread cap rubber layer with a tread running surface and an internal rubber layer underlying said tread cap rubber layer. Such tire constructions are well known to those having skill in such art.
The outer tread cap rubber layer is typically comprised of a rubber composition which provides resistance to tread wear during tire service, stiffness for tire handling purposes and traction for the road surface. Such typically desirable properties are well known to those having skill in such art.
The internal tread underlying rubber layer is typically comprised of a rubber composition which is less stiff than the outer tread cap rubber layer and less hysteretic to promote a reduced internal heat buildup during tire service to thereby provide a transition zone between the outer tread cap rubber layer and the tire carcass. Such typically desirable properties for a tire underlying rubber layer are well known to those having skill in such art.
However, some tires are intended to be provided for use as high performance tires which are expected to provide a challenge of handling and cornering ability. For this invention it is desired to increase the underlying tread rubber layer's stiffness to promote tire handling yet substantially maintain, without significantly increasing, the rubber's hysteresis property, or tendency of internal heat buildup during the tire's service.
While it is well known that a rubber composition's stiffness can normally be increased by increasing its reinforcing filler content, for example by increasing its rubber reinforcing carbon black content, it is also known that such methodology of increasing rubber stiffness through increased reinforcing filler can also be expected to adversely increase the rubber composition's hysteresis and thereby increase its internal heat buildup during tire service accompanied by a usually unwanted extent of temperature increase.
A challenge is then presented for providing a combination of both stiffness and acceptable hysteresis for the rubber composition of the underlying tread rubber layer.
A proposed response to such challenge is to provide an inclusion of syndiotactic-1,2-polybutdiene in the underlying tread rubber composition to increase the stiffness of the rubber composition.
However, it is envisioned that such inclusion of syndiotactic polybutadiene to increase the rubber stiffness would also promote increase the rubber's hysteresis property (increase its rebound property) and therefore promote increasing its internal heat build-up during service of the tire.
Accordingly, it is desired to increase the rubber stiffness by the inclusion of the syndiotactic polybutadiene while substantially maintaining the hysteresis property of the rubber composition (as evidenced by its rebound value) to therefore substantially maintain a consistent resistance to internal heat buildup during service of the tire.
Stiffness of the rubber composition promoted by the inclusion of the inclusion of the syndiotactic-1,2-polybutadiene which may, however tend to reduce the hysteresis of the rubber composition and thereby promote an increase in internal heat build-up. To counteract such phenomenon it is proposed to evaluate use of a diverse combination of carbon blacks.
Historically, syndiotactic polybutadiene has been used in various rubber compositions for various tire components for various purposes such as, for example, tire carcass and innerliner components. For example, see U.S. Pat. Nos. 5,307,850 and 6,956,093.
For such evaluation it is envisioned that such diverse combination of carbon blacks may comprise a combination of a relatively high rubber reinforcing rubber reinforcing carbon black with a high surface area (e.g. Iodine absorption value of at least 100 g/kg (ASTM D1510), to promote both rubber reinforcement and electrical conductivity for the syndiotactic polybutadiene-containing rubber combined with a significantly lower rubber reinforcing rubber reinforcing carbon black with a significantly lower surface area (e.g. Iodine absorption value of less than 50 g/kg) composition which would be expected to provide a combination of reduced rubber reinforcement and reduced electrical conductivity for the rubber composition.
In such manner, then the inclusion of the lower rubber reinforcing (lower surface area) carbon black would beneficially promote a reduction in hysteresis, and therefore a desirably reduced internal heat build up in the rubber composition, although with a reduction in reinforcing effect for the rubber composition and the inclusion of the higher rubber reinforcing (higher surface area) carbon black would beneficially promote a sufficient reinforcing effect for the rubber composition.
Representative of such high rubber reinforcing carbon black for purposes of this invention are rubber reinforcing carbon blacks which have an Iodine absorption value in a range of from about 100 to about 300 g/kg. Representative of such carbon blacks are, for example, according to their ASTM designations, N110, N121, N134, N220, N233, N234, N242, and N293.
Representative of such lower rubber reinforcing carbon black for purposes of this invention are rubber reinforcing carbon blacks which have an Iodine absorption value in a range of from about 10 to about 50, alternately about 25 to about 50, g/kg. Representative of such carbon blacks are, for example, according to their ASTM designations, N539, N550, N582, N630, N642, N650, N660, N683, N754, N762, N765, N774 and N787 and, also, N907, N907, N990 and N991.
Therefore, it is envisioned that a major portion of the diverse combination of rubber reinforcing carbon blacks will be comprised of at least one of said lower rubber reinforcing carbon blacks (lower surface rubber reinforcing carbon blacks).
It is believed that use of syndiotactic polybutadiene for a tread base rubber composition combined with such aforesaid combination of diverse rubber reinforcing carbon blacks is novel and a departure from past practice and that such combination is warranted for evaluation.
In the description of this invention, the term “phr” where used means “parts of material by weight per 100 parts by weight of rubber”. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “rubber composition” and “compound” may be used interchangeably unless otherwise indicated.
A volume electrical resistivity, which might sometimes referred to herein as electrical resistivity, may suitably be determined by DIN 53682 or ASTM Method D257-92 and reported herein as ohm-cm.
Dibutylthphalate (DBP) absorption values for carbon blacks is expressed in terms of cc/100 grams according to ASTM D2414. Nitrogen surface area (NSA) value, where referenced, is expressed in terms of square meters per gram (m2/g) according to ASTM D3037.